This invention relates generally to control systems for air conditioning/refrigeration systems.
Recent international legislation engendered by environmental concerns over the issues of global warming and ozone depletion have mandated the elimination of freons including those used in the multi-billion dollar air conditioning/refrigeration industry. Substitute refrigerants that have more beneficial environmental indices such as R134A (a replacement for R12 which is widely used in the automotive industry) have been proposed for use in conventional air conditioning/refrigeration systems but at the cost of incurring significant input power penalties, loss of capacity for given component size, and other problems such as lubricant incompatibility.
Most present air conditioning and refrigeration devices, especially those of small capacity typical of home and automotive air conditioners, use positive displacement compressor devices of relatively low pressure ratio (e.g. 4/1). These devices, which typically run at constant speed, pressure charge an accumulator or storage device with high pressure refrigerant for subsequent expansion (and cooling) in the evaporator heat exchanger. These systems cycle on and off to maintain the accumulator storage tank pressure. As the cooling load decreases and less refrigerant flow is required, the on-off cycle time becomes larger. Among the deficiencies with this approach are the fact that the energy efficiency during startup is significantly less than that of steady state running, the frequent cycling during all operating conditions can have deleterious impact on reliability, the variation of cooling with demand is considerable due to the charging and discharging of the accumulator, and, in fact, the load variation which can be accommodated with this type of operation is somewhat limited.
Subsequent improvements on this type of operation can be found in more modern systems which modulate compressor and, in some cases, evaporator and condenser fan speeds to obtain more nearly uniform operation and somewhat improved cycle turn down range and efficiency. These systems typically employed fixed or variable displacement piston, scroll, or wobble plate compressors, numerous examples of which can be found in the literature.
Recent technologies have been disclosed which use refrigerants with much higher specific volumes than the conventional R12 and R22 fluids. These systems, which are well suited to the utilization of small, high speed centrifugal compressors, require higher pressure ratios but much lower absolute pressures than normally encountered. U.S. Pat. No. 5,065,590 describes one such system. Typically, modulated systems such as the one mentioned in this patent control load change and variation of ambient conditions through hot gas bypass (highly inefficient) or through variation of compressor speed with compressor bypass (limited range of maximum to minimum load, again with less than optimum efficiency). However, these schemes and others not specifically described herein all are limited in their operation efficacy and performance optimization, both in power consumption and in range of operations, by the fact that all rely on the utilization of preprogrammed inputs (look-up tables) relating, not only to compressor mechanical constraints, manufacturing tolerances, mechanical condition, operational characteristics, and limiting parameters, but also to refrigerant fluid thermodynamic and aerodynamic parameters.
Given the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an air conditioning/refrigeration system that is able to use an environmentally safe refrigerant without the losses in efficiency expected of prior art systems.
It is another object of the present invention to provide an air conditioning/refrigeration control system that provides high efficiency over the entire load range.
It is yet another object of the present invention to provide an air conditioning/refrigeration control system adapted to use a compressor, for example, a single stage centrifugal compressor, a two stage centrifugal compressor, an axial/centrifugal compressor, or a mixed flow combination compressor, that can be modulated through a speed range for mid- to high-load ranges and can cycle on and off for mid- to low-load ranges.
It is a further object of the present invention to provide a control system for an air conditioning/refrigeration system that enables surge-free operation during steady state and during load changes.
It is another object of the present invention to provide an air conditioning/refrigeration control system that minimizes compressor acceleration time from stop to full load.
It is yet another object of the present invention to provide an air conditioning/refrigeration system that has the ability to use other refrigerants in the same system.
It is a further object of the present invention to provide a control system for an air conditioning/refrigeration system that includes an intrinsic diagnostic capability to detect deteriorated and/or damaged components.
It is yet another object of the present invention to provide an air conditioning/refrigeration control system that maximizes cycle efficiency through control of degree of superheat during steady state and transient operation.